The present invention relates to a method of moulding and curing tires for vehicle wheels, comprising the following steps: disposing a tire on a toroidal support, the outer surface of which substantially mates an inner surface of the tire; closing the tire and the toroidal support inside a moulding cavity defined in a vulcanization mould, said moulding cavity having walls the shape of which match that of an outer surface of the tire when vulcanization has been completed; pressing the tire against the moulding cavity walls; and, transmitting heat to the tire to cause a molecular cross-linking of same.
The invention also relates to an apparatus for moulding and curing tires for vehicle wheels, comprising: a toroidal support arranged to engage a tire, said toroidal support having an outer surface substantially mating an inner surface of the tire; a vulcanization mould comprising at least two cheeks axially movable between an open condition in which they are mutually spaced apart to enable said toroidal support carrying a tire to be introduced thereinto, and a closed condition in which they are disposed in a mutual side-by-side relationship for enclosing the toroidal support and tire, within a moulding cavity confined by inner walls of the mould, the shape of which match an outer surface of the cured tire; pressing devices for pressing the outer surface of the tire against the inner walls of the mould; and, heating devices for transmitting heat to the tire enclosed between the moulding cavity and the toroidal support.
In a tire production cycle it is provided that, after a manufacturing process in which the different tire components are made and/or assembled, a moulding and curing process should be carried out for the purpose of stabilizing the tire structure to a given geometric configuration, characterized by a particular tread pattern.
To this aim, the tire is introduced into a vulcanization mould usually comprising a pair of cheeks adapted to be axially moved close to each other, which are arranged to operate on the tire bead and sidewalls, and at least one crown consisting of circumferentially distributed sectors susceptible of being radially moved close to each other so as to operate at the tire tread band. In more detail, cheeks and sectors are mutually movable between an open condition, in which they are spaced apart from each other for enabling loading of the tires being processed, and a closed condition in which they define a moulding cavity the geometric configuration of which is the same as the outer surfaces of the tire to be obtained.
In one of the most widespread moulding methods, it is provided that a vulcanization bladder of elastomer material filled with steam and/or another high-temperature and high-pressure fluid should be inflated at the inside of the tire enclosed in the moulding cavity. In this manner, the tire is conveniently pushed against the inner walls of the moulding cavity and stabilized to the geometric configuration imposed to it, following a molecular cross-linking to which the elastomer material, of which it is made, is subjected, due to heat transmitted by the fluid through the bladder and by the mould walls.
Also known are moulding methods in which, instead of an inflatable vulcanization bladder, a rigid toroidal support, having the same configuration as the inner surface of the tire to be obtained, is arranged within the tire.
Such a method is disclosed, for example, in the European Patent EP 242, 840, in which a rigid toroidal support is employed for imposing the shape and defining the tire enclosed in the mould. According to the above patent disclosure, the different coefficient of thermal expansion between the toroidal metal support and the raw elastomer material of which the tire is made is utilized for achieving an appropriate moulding pressure.
In conclusion, in such a prior art method, the assembly of the parts forming the mould and the toroidal support define a closed space in the moulding cavity which is exactly shaped as the whole geometric configuration of the tire. In this way, both the outer surfaces and the inner surfaces of the tire are maintained in contact with rigid portions of the moulding and curing apparatus. In other words, all parts of the apparatus that are intended for setting the final tire geometry are rigid parts, in contrast with the methods using an inflatable vulcanization bladder that, as known, constitutes a deformable portion of the mould.
It is the Applicant""s feeling that, at the present state of the art, both methods using an inflatable vulcanization bladder and methods using a rigid toroidal support during the tire vulcanization have some problems.
With reference to the methods using an inflatable bladder, it is, in fact, to note that the bladder deformability can easily give rise to geometric and/or structural imperfections in the tire due to possible distortions suffered by the bladder, following an unbalanced expansion, for example, and/or due to friction phenomena generated between the outer surfaces of the bladder and the inner surfaces of the green tire.
Since the bladder also locks the tire beads against the corresponding mould portions, the bladder deformability makes it difficult to reach sufficiently high pressures for bead locking. Thus, undesired misalignments of the beads relative to the geometric axis of the tire may occur, giving then rise to distortions of the whole tire structure. In addition, an insufficient pressure for bead locking may cause formation of flashes at the beads, due to leakage of the elastomer material between the bladder and the mould, above all at the starting instants of the vulcanization process.
The vulcanization bladder needs use of significant amounts of steam, since the whole inner volume of the bladder inflated in the mould cavity is to be filled up. In addition, the bladder constitutes an obstacle to transmission of heat to the tire by the steam.
On the other hand, use of a rigid toroidal support instead of the inflatable vulcanization bladder makes it necessary to carry out a very precise and difficult checking of the volumes of the material employed in manufacturing the tire.
In addition, it is presently impossible to impose an appropriate radial and/or circumferential expansion to the tire, for achieving desired preloading effects in the reinforcing structures employed in the tire manufacture, for example.
Furthermore, even with the aid of the rigid toroidal support, achievement of a correct and efficient heat transmission to the inside of the tire is rather difficult.
U.S. Pat. No. 1,798,210 discloses a curing method according to which a previously-manufactured green tire is fitted on a toroidal support of vulcanized rubber, to be then closed in the moulding cavity defined in a vulcanization mould. The toroidal support is completely hollow and is such shaped and sized that it cooperates with the inner walls of the moulding cavity to carry out a hermetic seal at the inner circumferential edges of the tire. The toroidal support sizes however are lower than the inner sizes of the green tire, so as to define a gap extending from one bead to the other between the outer surfaces of the toroidal support and the inner surfaces of the green tire. After carrying out closure of the mould, hot water and/or another hot fluid under pressure is admitted to the toroidal support, which fluid reaches the above described gap through openings formed in the support for fulfilling all functions required for tire moulding and curing.
In this curing process, however, manufacturing of the tire directly an the toroidal support to be introduced into the vulcanization mould together with the tire is neither provided nor allowed.
In addition, since the toroidal support must necessarily be lower in size than the inner size of the tire, structural defects resulting from an imperfect centering and/or uncontrolled movements or distortions to which the tire is submitted on its being closed in the moulding cavity, may easily arise.
The Applicant has become aware of the fact that important improvements may be achieved if admission of working fluid, for carrying out tire moulding and/or heat supply for vulcanization, takes place at the inside of a gap which is formed between the toroidal support and the green tire only following an expansion imposed to the tire by application of pressure. A method and an apparatus conceived on the basis of this principle are the object of European Patent Application No. 98830473.9 in the name of the same Applicant.
In accordance with the present invention, it has also been found that important improvements may be achieved as regards tire expansion, with advantageous effects in terms of quality of the final product, if, concurrently with the expansion imposed to the tire, the inner circumferential edges of said tire are substantially disengaged from the toroidal support and allowed to move in a controlled manner relative to the latter.
In more detail, it is an object of the invention to provide a method of moulding and curing tires for vehicle wheels, characterized in that said pressing step takes place concurrently with an expansion imposed to the tire, preferably by admission of a fluid under pressure to at least one gap for fluid diffusion created between the outer surface of the toroidal support and the inner surface of the tire, an axial moving apart of the inner circumferential edges of the tire from the toroidal support being carried out concurrently with the tire expansion.
Preferably the axial moving apart of each inner circumferential edge of the tire from the toroidal support is carried out by axially moving backwardly at least one radially inner annular portion of a cheek being a part of said vulcanization mould and that contacts on the respective inner circumferential edge of the tire.
It is also preferably provided that concurrently with the tire expansion, side portions of the tire move to come into contact with fixed portions of the cheeks. This movement of the tire side portions preferably begins before the axial moving apart step of said inner circumferential edges. Advantageously, axial moving apart of the inner circumferential edges of the tire is carried out to an extent correlated with the expansion imposed to tire.
In accordance with a preferred embodiment of the invention, before said pressing step a tire preforming step is carried out by preliminary admission, between said outer surface of the toroidal support and the inner surface of the tire, of a working fluid to a pressure lower than that achieved during the pressing step.
The radial expansion resulting from the pressing step preferably involves an increase in the tire circumference between 1.5% and 3.5%, measured at the equatorial plane of the tire.
The diffusion gap during the pressing step preferably has a size between 3 mm and 14 mm measured between the inner surface of the tire and the outer surface of the toroidal support at the equatorial plane of the tire.
In a preferred embodiment, at least the preliminary admission of fluid under pressure for execution of the preforming step takes place through feed channels formed in the toroidal support and opening onto the outer surface thereof.
It is also preferably provided that during the preliminary admission of working fluid the tire should be sealingly engaged at its inner circumferential edges between the walls of the moulding cavity and the outer surface of the toroidal support, to hermetically delimit the diffusion gap at the inner circumferential edges of the tire.
Advantageously, heat supply preferably takes place by admission of a heating fluid to said diffusion gap. This heating fluid may constitute, or at least be part of, the same fluid under pressure employed for carrying out the pressing step.
It may be also conveniently provided that before admission of fluid under pressure the inner tire surface should adhere, substantially over the whole extension thereof, to the outer surface of the toroidal support, said diffusion gap being created following expansion of the tire.
Preferably, admission of fluid under pressure takes place to an upper portion of the moulding cavity communicating with a lower portion of the cavity through said diffusion gap.
Also preferably carried out concurrently with said admission step, is a step of drawing the fluid out of the lower portion of the moulding cavity, so as to create a pressurized fluid stream in the diffusion gap.
Conveniently, the step of disposing the tire on the toroidal support is accomplished by directly manufacturing the tire on the toroidal support.
In accordance with a further aspect of the invention, before the pressurized fluid admission, a treatment of the inner surface of the tire is carried out for preventing permeation of the fluid under pressure through the elastomer material forming the green tire. In more detail, a pre-cured liner is provided on the inner surface of the tire. Said pre-cured liner can be advantageously directly formed on the toroidal support during a preliminary step of the tire manufacture on the toroidal support.
In accordance with the invention, said method is put into practice by an apparatus for moulding and curing tires for vehicle wheels, characterized in that each of said cheeks comprises a radially outer annular portion and a radially inner annular portion acting at a respective inner circumferential edge of the tire and axially movable relative to said outer annular portion in a closed condition.
Preferably, said pressing devices are comprised of channels for feeding a fluid under pressure which are formed through the toroidal support and open onto the outer surface of the latter. In more detail, the radially inner annular portions of said cheeks are movable relative to said outer annular portions between a first position in which they cooperate with inner circumferential portions of the toroidal support to hermetically engage the tire in a sealing manner at respective inner circumferential edges thereof, and a second position in which they are spaced apart from the inner circumferential portions of the toroidal support.
Advantageously, the outer surface of the toroidal support and the inner walls of the mould in said moulding cavity delimit a holding space for the tire which, when the mould is closed, has a greater volume than the volume taken up by the tire.
It is also provided that the outer surface of the toroidal support should have a smaller diameter than the diameter of the inner surface of the cured tire.
The feed channels preferably open into at least one diffusion gap for the fluid under pressure which is defined between the outer surface of the toroidal support and the inner surface of the tire being processed.
Conveniently, the toroidal support has at least one centering shank to be engaged in a centering seating arrangement in the mould for centering the toroidal support and tire within the moulding cavity. This centering shank extends for example along a geometric axis common to said toroidal support, said tire being processed and said moulding cavity.
According to a further feature of the invention, the heating devices comprise at least one duct for sending a heating fluid to the feed channels. The heating fluid may consist of the same fluid under pressure admitted by the pressurized fluid feeding devices.
It may be also conveniently provided that the toroidal support should have an elastically yielding structure in an axial direction, at least at regions corresponding to inner circumferential edges of the tire.